In a gas and steam turbine installation, the heat contained in the expanded working medium or heating gas is used for generating steam for the steam turbine. The transfer of heat is carried out in a waste-heat steam generator connected downstream of the gas turbine, in which waste-heat steam generator a number of heating surfaces are usually disposed for preheating water, for generating steam and for superheating steam. The heating surfaces are connected to the water-steam circuit of the steam turbine. The water-steam circuit usually comprises multiple, e.g. three, pressure stages, whereby each pressure stage can have an evaporator heating surface.
Multiple alternative layout concepts may be considered for the steam generator connected as a waste-heat steam generator downstream of the gas turbine on the heating-gas side, namely a layout as a continuous-flow steam generator or a layout as a circulation steam generator. In a continuous-flow steam generator, the heating of steam-generating pipes provided as evaporator pipes leads to an evaporation of the flow medium in the steam-generating pipes in a single pass. In contrast to this, in a natural-circulation or forced-circulation steam generator, the circulated water is only partially evaporated in one pass through the evaporator pipes. The water which is not evaporated is, following separation of the generated steam, fed into the same evaporator pipes again for further evaporating.
In contrast to a natural-circulation or forced-circulation steam generator, a continuous-flow steam generator is not subject to any pressure limitation, so live-steam pressures far above the critical pressure of water (PCri≈221 bar)—where there are only small differences in density between a liquid-like and a steam-like medium—are possible. A high live-steam pressure promotes a high thermal efficiency and consequently low CO2 emissions in a fossil-fuel fired power station. Also, a continuous-flow steam generator has, in comparison with a circulation steam generator, a simple design and can consequently be produced at particularly low cost. The use of a steam generator, designed in accordance with the continuous-flow principle, as a waste-heat steam generator in a gas and steam turbine installation is therefore particularly suitable for achieving a high degree of overall efficiency of the gas and steam turbine installation with a simple design.
Such a waste-heat steam generator can be technically implemented particularly easily where the heating gas supplied to the steam generator from the gas turbine flows through the gas duct in a vertical direction, in particular from bottom to top. In principle, two possible designs can be considered for the flow-medium and heating-gas connections of the steam-generating pipes which form the evaporator throughflow heating surface: either the flow medium flows through the steam-generating pipes laid inside the gas duct in a cross-flow or counterflow, i.e. the flow medium flows through each heating surface pipe in successive passes through the gas channel across the gas flow, hence the name cross-flow circuit. The horizontal pipe elements leading from one side of the gas channel to the other side are connected to one another via redirecting elements in such a manner that the flow passes through them in succession in a vertical direction counter to the direction of flow of the gas, hence the name counterflow circuit. Overall, it is thus a mixed form of cross-flow and counterflow circuit. The cross-flow character is immaterial to the arguments below. This circuit will therefore be referred to below only as a counterflow circuit. It is generally known that an evaporator heating surface in a counterflow circuit is problematical in terms of the stability of the flow. In particular, an even distribution of the flow over all the parallel pipes of the evaporator heating surface requires a technical outlay.
An alternative to the counterflow circuit is provided by the so-called parallel-flow circuit, in which the flow through the steam-generating pipes is a cross/parallel flow. In this circuit, the horizontally routed pipe elements are connected to one another, as in the previously described cross-flow circuit, via redirecting elements, except that now the flow passes through them in succession in a vertical direction, in the direction of flow of the gas, hence the name parallel-flow circuit. Overall, this is thus a mixed form of cross- and parallel-flow circuit. The cross-flow character is immaterial to the arguments below. This circuit will therefore be referred to below only as a parallel-flow circuit. A parallel-flow circuit necessitates the use of comparatively large heating surfaces, the production and assembly of which involve a substantial outlay.
From EP 0 425 717 A, a steam generator is known which has the specified advantages of a continuous-flow steam generator. Its evaporator throughflow heating surface is designed as a combination of counterflow and cross-flow circuit, in that a number of pipe sections are connected in a counterflow direction, while a number of further pipe sections are connected in a parallel-flow direction. This type of circuitry enables a greater degree of flow stability to be achieved than in the case of a pure counterflow circuit. Also, the high outlay required on equipment and apparatus where a pure counterflow circuit is used can be reduced.
A fundamental problem in steam generators of this type of design can be temperature distortions, that is temperature differences at the outlets of adjacent steam-generating pipes connected in parallel on the flow-medium side, which can lead to pipe bursts or other damage. In order to avoid such temperature distortions, continuous-flow steam generators can be designed for particularly low mass-flow densities of the flow medium. However, this limits flexibility in the choice of design parameters for the steam generator.